In order to reproduce matching colors across multiple devices, such as a printer and a monitor, a color conversion table is generally created for each device. One method of creating a color conversion table for a printer is to control the printer to print color patches on paper and to measure the color of each patch with a colorimeter in order to obtain numerical values for each color in the L*a*b* color space. Next, a color conversion table specifying correlations between numerical values in the RGB color space and numerical values in the L*a*b* color space is created based on the relationship between known RGB values and measured values for color patches corresponding to the known RGB values. This color conversion table is then used to perform seamless color matching between devices.
However, using a color conversion table created according to the conventional method described above can potentially lead to the use of colors that are darker than the color representing black.
The graph shown in FIG. 7 illustrates RGB color gamuts defined within the L*a*b* color space according to the conventional color conversion table. For simplicity, the gamuts are rendered in the L*b* plane in FIG. 7. As shown in FIG. 7, the color gamut for blue (B) defined by the conventional color conversion table has a dark region that extends lower than the lightness (L* value) of black. It is possible to produce a color conversion table of this type when unconditionally using colorimetric values obtained by measuring color patches.
Thus, if Lab values are converted to RGB values using this color conversion table, the resulting color gamut will produce darker colors than black. Consequently, when an image is outputted based on RGB values obtained from this color conversion table, the resulting image having dark blue regions rendered in darker colors than black may appear unnatural.
FIG. 8 shows gradations from blue to black and from black to blue that are outputted from a printer based on the RGB values obtained using the conventional color conversion table, and the measured values obtained from these gradations. As shown in FIG. 8, if black is rendered with a higher lightness than that in the dark blue region, black areas formed adjacent to dark blue areas will appear to have a sheen. In the gradations shown in the upper part of FIG. 8, the shininess of the black region has been emphasized to clearly illustrate the phenomenon, but the shine may not appear as extreme as that shown in FIG. 8 in the actual printing results.